The present invention relates to a method and an apparatus used for repeating and resending the digital transmission signal for television broadcasting, or in particular to a resending method and apparatus having the function of restricting the resending of an adversely-affecting signal such as noises.
In the live TV broadcast of a marathon or the like, a movable portable broadcasting link (such as FPU (field pickup unit)) is used, so that the television signal picked up by a camera is transmitted from the FPU to an airborne resending apparatus mounted on a helicopter or balloon and then from the resending apparatus to a broadcasting station.
The orthogonal frequency division multiplexing (OFDM) has recently begun to be used as a modulation scheme for transmission of the digital signal like the television signal.
In the OFDM modulation scheme, a multiplicity of carriers are used, and a guard interval period is added at the transmitting end to reduce the effect of a delayed wave which may be mixed, and therefore has a high resistance to an environment which may develop a fading.
In the digital signal transmission according to the OFDM modulation scheme, the information is digitized and the error correcting process is used. In the case of the selective fading with the level of a part of the frequency band reduced, the carrier in the band with no level reduction can be normally received. The data of the carrier lost by the selective fading, therefore, can be restored by the error correction. Also, even in the case where the signal delayed by reflection or the like is mixed, the signal is buffered by the guard interval period and not easily deteriorated.
In the OFDM transmission scheme, despite this high resistance, the ratio of noises to the signal increases if the received electric field level is decreased to less than the critical value. In such a case, the error of the carrier of normal level also increases to such an extent that the error of the whole carrier as well as a specified deteriorated carrier becomes difficult, and the normal transmission may become impossible.
The critical value is inversely proportional to the data amount transmitted. In the 64 QAM convolution correction 5/6 mode having the transmission rate of as much as 60 Mbps, for example, the critical carrier-to-noise (CN) ratio is about 24 dB, which requires the limit of the received electric field of not less than about −73 dBm. In the QPSK convolution correction 1/2 mode having a transmission rate as small as 12 Mbps, on the other hand, the critical CN ratio is about 6 dB, so that the normal transmission is possible with the receiving electric field limit of about −91 dBm or more.
As described above, the receiving electric field level has a great effect not only on the transmissible data amount and the reliability thereof but also on the mixing ratio of the reflected wave and the delay time thereof at the same time.
In the state generally called a “perspective state” free of buildings which otherwise might block the radio wave between the transmitting point and the receiving point, the basic receiving field level (intensity) at a receiving end 1 is determined by the radio wave's frequency and the distance between the transmitting point and the receiving point.
As shown in FIG. 9, for example, an area A is farthest from a repeater (receiving end) 1 up in the air as compared with an area B having a building 5 constituting a blocking object and an area C having a building 6 constituting a blocking object. In view of the fact that the area A is in perspective state, however, the location of a FPU (transmitting end) 2, if any, in the area A increases the field at the receiving end 1 to middle to high level and therefore realizes a stable signal transmission.
In what is generally called the over-the-horizon state where a building or the like blocking the radio wave exists between the transmitting point and the receiving point, the receiving field level at the receiving end 1 is lower than in the perspective state. In this case, the amount by which the signal decreases at the receiving end 1 as compared with the transmitting point is about 10 to 20 dB, depending on the size of the blocking building or the presence or absence of a path through which the radio wave is reflected and can reach the receiving point. In some cases, the amount of reduction is not less than 20 dB.
Assume, for example, that a FPU (transmitting end) 2 is located in the area B in which a large building 5 blocking the electric wave exists between the transmitting point and the receiving point. In spite of the fact that the area B is nearer to the repeater (receiving end) 1 than the area A, the radio wave W2 from the FPU (transmitting end) 2 is blocked by the building 5 and fails to reach the repeater (receiving end) 1, with the result that the field level at the receiving end 1 is reduced to a low level or zero.
In the case where the FPU (transmitting end) 2 is located in the area C as shown in FIG. 12, as another example, the over-the-horizon state prevails due to the building 6, so that the radio wave W5 directly transmitted from the FPU (transmitting end) 2 fails to reach the receiving end 1, while the radio wave W6 transmitted from the FPU (transmitting end) 2 and reflected on the wall surface of the building 5 in the area B reaches the repeater (receiving end) 1. As a result, the field at the receiving end 1 may be reduced to middle or low level.
In the case where the FPU (transmitting end) 2 is located between the area B and the area C (between buildings) as shown in FIG. 11, on the other hand, the direct wave W3 passing through a path in the perspective state and the wave W4 reflected from the building 5 may reach the repeater (receiving end) 1.
The OFDM transmission scheme generally has a high resistance to the radio wave containing the reflected wave (i.e. delayed wave), as described above. In the case where the radio wave contains a reflected wave having a delay time longer than the guard interval period, however, the noise ratio increases and normal receiving operation becomes impossible even when the field level is high at the receiving end 1.
The live broadcasting of a marathon race, for example, requires transmission of video data while moving the FPU along a course as long as 42 km. Various topologies and buildings exist along the course, most of which are liable to develop a transmission fault as shown in FIGS. 10 to 12. Depending on the frequency range of the radio wave used, in the case of the digital FPU in 7 GHz band, for example, the receiving signal not higher than −97 dBm is buried under noises. Even in the case where the gain of the amplifier at the receiving end 1 is increased, only the noises are generated and no correct transmission becomes possible.
In the prior art, therefore, as shown in FIG. 13, for example, a resending apparatus 3 called a gap filler is arranged at a high place such as on the roof of a building. The gap filler 3 receives the radio wave from the FPU (transmitting end) 2 on the roof of the building or the like, and amplifying the received signal, resends it toward the repeater (receiving end) 1.
An example of the gap filler is disclosed in JP-A-2002-94482.